The need to present an undistorted image to the viewer of a cathode ray tube (CRT) presents problems to the designer of the control apparatus. The image is formed by scanning an electron beam over the tube face, a face which may be curved, but does not have the same radius as the apparent radius of the electron beam. In a single beam tube this results in what is known as pin-cushion distortion, a pinching inwards of the scan at the centers of the edges of the image, and an elongation at the corners. Because the gun in a single beam tube is aligned along a central axis of the tube, pin-cushion distortion is symmetrical about a center point of the screen.
Distortion of the image is aggravated in a three beam color tube as, of necessity, the three guns are displaced from each other and each color produces its own displaced pin-cushion raster on the screen. The three beams may, without correction, converge on one spot in the center of the screen but equal horizontal and vertical deflection of the three beams results in a divergence of the beams as well as a displaced pin-cushion effect.
Consequently two distortions have to be corrected before an image is acceptable to a viewer, first the pin-cushion effect and secondly the convergence of the three beams so that they present an aligned image.
Copending U.S. patent application Ser. No. 860,402, filed Dec. 13, 1977 by M. H. Hallett, M. J. Heneghen and B. R. Sowter and assigned to the assignee of this application, describes control apparatus for storing correction factor signals in digital form for a CRT screen divided into a plurality of discrete areas. The correction factor signals are derived for each area by an operator converging first a red and green pattern to give a yellow pattern and then the yellow pattern with a blue pattern to give a white pattern. During each refresh operation of the CRT display, the correction signals thus stored are fetched from memory in synchronism with the movement of the CRT beam to the respective areas of the CRT screen, converted to analog currents and applied to convergence coils of the CRT. In the embodiment described in said copending application there are sixty four discrete areas of the screen and the correction factor signals are derived by the operator as above described from each area separately. This can be a long and tedious process.